Method for reducing water-soluble impurities in benzene-extracted caprolactam solution

Abstract

The invention discloses a method for reducing water-soluble impurities in a benzene-extracted caprolactam solution. The method comprises the following steps: mixing a benzene-hexyl solution obtained in a benzene extraction process with a benzene-containing solvent, and carrying out secondary extraction; carrying out primary distillation on the light phase obtained by secondary extraction to distill out n-hexane for cyclic utilization; carrying out secondary distillation on the residual benzene-hexane solution after the primary distillation to distill out benzene for cyclic utilization; carrying out water back extraction on the residual benzene-containing caprolactam solution after secondary distillation; carrying out benzene distillation on a light phase obtained by water reverse extraction and recovering benzene, and carrying out benzene stripping and ion exchange on a heavy phase obtained by water reverse extraction. Compared with the existing method for washing the benzene-extracted caprolactam solution by using dilute alkaline water, the method disclosed by the invention has the advantages that the content of water-soluble impurities in the caprolactam solution and the conductivity of the benzene-caprolactam solution are greatly reduced, an alkaline washing procedure is omitted, the concentration of the caprolactam after a reverse extraction procedure is improved, and the load of a distillation procedure is reduced.

Description

Technical Field

[0001] The invention relates to the field of chemical mass transfer and separation, and in particular to a method for reducing water-soluble impurities in a benzoate-caprolactam solution. Background Art

[0002] Caprolactam is an important chemical raw material, but its production process produces a large number of impurities, mainly including water-soluble impurities such as ammonium sulfate and a small amount of organic impurities such as alcohols, ketones, oximes, etc. At present, the removal of water-soluble impurities from caprolactam in the market is mainly done by adding benzene for extraction.

[0003] According to the different solubilities of caprolactam and water-soluble impurities in benzene and water, benzene is used as an extractant to extract crude caprolactam, a benzene solution of caprolactam is obtained as a light phase, and an impurity-containing aqueous solution is obtained as a heavy phase, thereby separating the water-soluble impurities in the crude caprolactam.

[0004] However, water has a certain solubility in the benzene-caprolactam system. The benzene-caprolactam solution obtained through the benzene extraction process can only remove part of the water-soluble impurities, which limits the product quality. Therefore, it is necessary to find a more effective method for removing water-soluble impurities. Summary of the invention

[0005] In view of this, the present invention provides a method for reducing water-soluble impurities in a benzene-extracted caprolactam solution, so as to solve the problem of high water-soluble impurities of caprolactam in the extracted material in the prior art.

[0006] In order to achieve the above object, the present invention provides a method for reducing water-soluble impurities in a phenyl caprolactam solution, comprising the following steps:

[0007] 1) sending the benzene-hexyl solution and n-hexane or a mixed solvent of n-hexane and benzene obtained in the primary extraction process into an extractor for secondary extraction; the benzene-hexyl solution comprises benzene and caprolactam and impurities dissolved therein, and the water-soluble impurities are mainly ammonium sulfate;

[0008] 2) sending the light phase obtained from the secondary extraction in step 1) to a solvent for primary distillation to distill out the n-hexane for recycling;

[0009] 3) performing secondary distillation on the benzene-hexyl solution remaining after the primary distillation in step 2) to distill out the benzene for recycling;

[0010] 4) subjecting the benzene-containing caprolactam solution extracted from the secondary distillation tower bottom in step 3) to water back extraction;

[0011] 5) The light phase obtained by water stripping in step 4) is subjected to benzene distillation to recover benzene, and the heavy phase obtained by water stripping is sent to benzene stripping, ion exchange, hydrogenation, three-effect evaporation, separation, flash distillation, light removal, and distillation steps in sequence.

[0012] Preferably, in step 1), the mass content ratio of benzene to caprolactam in the benzene-hexyl solution is 2.5 to 8:1.

[0013] Preferably, in step 1), the mass content of the water-soluble impurity ammonium sulfate in the benzene-hexyl solution is 0.1-10%.

[0014] Preferably, in step 1), the mixed solvent comprises benzene and n-hexane.

[0015] Preferably, in step 1), the mass ratio of benzene to n-hexane in the mixed solvent is 0.1 to 1:1.

[0016] Preferably, in step 1), the temperature of the secondary extraction is 40° C. and the pressure is 1 kPa.

[0017] Preferably, in step 1), the mass ratio of the mixed solvent to the benzene-hexyl solution is 1:2-10.

[0018] Preferably, in step 2), the operating temperature of the primary distillation is 65-75°C.

[0019] Preferably, in step 3), the operating temperature of the secondary distillation is 75-95°C.

[0020] Preferably, in step 4), the mass ratio of benzene to caprolactam in the caprolactam solution is 1:1-4.

[0021] Preferably, in step 4), the mass ratio of the benzene-hexyl solution added to the water stripper to water is 6 to 4:1.

[0022] Preferably, in step 4), the operating temperature of the water stripper is 40° C. and the operating pressure is 1 kPa.

[0023] Preferably, in step 5), the light phase is crude benzene having a caprolactam content of less than 0.1 wt%.

[0024] Preferably, in step 5), the heavy phase is a caprolactam-water solution having a caprolactam content of 25 wt% to 40 wt%.

[0025] The beneficial technical effects of the present invention are:

[0026] 1. The method of the present invention reduces the content of water-soluble impurities in caprolactam, thereby reducing the conductivity of the stripping material and eliminating the alkali washing process.

[0027] 2. The method of the present invention reduces the water consumption in the stripping process by 10 wt%, increases the caprolactam concentration of the stripping material from 30 wt% to 40 wt% to 55 wt%, and thus reduces the load of the distillation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The present invention is a schematic flow diagram of a method for reducing water-soluble impurities in a benzoate-caprolactam solution. DETAILED DESCRIPTION

[0029] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following is a method for reducing water-soluble impurities in a phenyl caprolactam solution according to the present invention in combination with a preferred embodiment, and its specific implementation, features and performance are described in detail as follows. In the following description, different "one embodiment" or "embodiment" does not necessarily refer to the same embodiment. In addition, specific features, structures or characteristics in one or more embodiments may be combined in any suitable form.

[0030] The following materials or reagents, unless otherwise specified, were purchased from the market.

[0031] like Figure 1 As shown, some embodiments of the present invention provide a method for reducing water-soluble impurities in a benzoic acid caprolactam solution, comprising the following steps:

[0032] 1) sending the benzene-hexyl solution and n-hexane or a mixed solvent of n-hexane and benzene obtained in the primary extraction process into an extractor for secondary extraction; the benzene-hexyl solution comprises benzene and caprolactam and impurities dissolved therein, and the water-soluble impurities are mainly ammonium sulfate;

[0033] 2) The light phase obtained from the secondary extraction is sent to a solvent for primary distillation to evaporate the n-hexane for recycling;

[0034] 3) The remaining benzene-hexyl solution after the primary distillation is subjected to secondary distillation to distill benzene for recycling;

[0035] 4) The benzene-containing caprolactam solution extracted from the secondary distillation tower bottom is subjected to water back extraction;

[0036] 5) The light phase obtained by water stripping is subjected to benzene distillation to recover benzene, and the heavy phase obtained by water stripping is sequentially sent to benzene stripping, ion exchange, hydrogenation, triple-effect evaporation, separation, flash distillation, light removal, and distillation steps to obtain caprolactam with a mass content of 99.99%. The benzene stripping, ion exchange, hydrogenation, triple-effect evaporation, separation, flash distillation, light removal, and distillation steps can be any benzene stripping, ion exchange, hydrogenation, triple-effect evaporation, separation, flash distillation, light removal, and distillation steps in the prior art, which will not be repeated here.

[0037] In the above technical scheme, the present invention uses n-hexane or a mixed solvent of n-hexane and benzene to perform secondary extraction on the benzene-hexane solution. Since the solubility of water in the n-hexane-benzene-caprolactam mixed solution is lower, water-soluble impurities are precipitated together with water at the bottom layer, so that the water-soluble impurities in the upper benzene-n-hexane-caprolactam solution are further reduced. The n-hexane and benzene are successively distilled out by distillation, so that the benzene content in the obtained caprolactam solution is reduced from 80wt% to 50wt%-75wt%, the water consumption of the stripping process is reduced by 10wt%, and the caprolactam concentration of the stripping material is increased from 30wt% to 40wt%-55wt%, thereby reducing the load of the distillation process.

[0038] In the above step 1), the mass content ratio of benzene to caprolactam in the benzene-hexyl solution is 2.5:1 to 8:1. If the mass content ratio is less than 2.5:1, the benzene content is low, which means that the amount of benzene added in the benzene extraction process is small and the extraction is incomplete; if the mass content ratio is greater than 8:1, the benzene content is high, and more steam is required for refining crude benzene.

[0039] In the above step 1), the mass content of the water-soluble impurity ammonium sulfate in the benzene-hexyl solution is 0.1-10%. If the mass content is greater than 10%, the ammonium sulfate content is too high and a large amount of ammonium sulfate will be lost, which is poor in economic efficiency; if the mass content is less than 0.1%, ammonium sulfate impurities exist in the production process of crude caprolactam, and the impurities need to be removed in the refining process of caprolactam. Due to the limitation of the production process, the impurities must exist in the crude caprolactam, i.e., the amide oil in the figure.

[0040] In the above step 1), the mixed solvent comprises benzene and n-hexane.

[0041] In the above step 1), the mass ratio of benzene to n-hexane in the mixed solvent is 0.1 to 1:1. If the mass ratio is greater than 1:1, a larger amount of mixed solvent is required when the n-hexane content is less than 50%, the corresponding extraction tower volume is larger, and the equipment cost is higher; if the mass ratio is less than 0.1:1, the n-hexane content is higher than 90%, the subsequent equipment requirements for crude n-hexane purification are higher, and the equipment cost is high.

[0042] In the above step 1), the temperature of the secondary extraction is 20-40° C. and the pressure is 0-2 kPa. When the temperature and pressure are too low, the extraction efficiency is low, and when the temperature and pressure are too high, it will cause flooding.

[0043] In the above step 1), the mass ratio of the mixed solvent to the benzene-hexyl solution is 1:2 to 10. If the mass ratio is greater than 1:2, the mixed solvent content is too high, which will increase the burden of the primary distillation and the secondary distillation; if the mass ratio is less than 1:10, the mixed solvent content is too low to play an extraction role.

[0044] In the above step 2), the operating temperature of the primary distillation is 65-75°C. When the temperature is lower than 65°C, n-hexane cannot be distilled out, and when the temperature is higher than 75°C, more benzene is entrained in the gas phase.

[0045] In the above step 3), the operating temperature of the secondary distillation is 75-95°C. When the temperature is lower than 75°C, benzene cannot be distilled out, and when the temperature is higher than 95°C, flooding is likely to occur.

[0046] In the above step 4), the mass ratio of benzene to caprolactam in the caprolactam solution is 1:1 to 4. When the benzene content is lower than 50wt%, the concentration of caprolactam is higher than 50wt%, more water needs to be added to the water stripper, the required extractor volume is large, and the equipment cost is high; when the benzene content is higher than 80wt%, the amount of crude benzene extracted from the top of the water stripper is large, the amount of caprolactam dissolved in the crude benzene increases, and the loss of caprolactam increases.

[0047] In the above step 4), the mass ratio of the benzene-hexyl solution to water added to the water stripper is 6 to 4:1. If the mass ratio is less than 4:1, the volume of the extractor required will increase when the water content is too high, and the equipment cost will increase; if the mass ratio is greater than 6:1, when the water content is too low, the extraction is incomplete, and the amount of caprolactam dissolved in the crude benzene extracted from the top of the water stripper increases, which increases the loss of caprolactam.

[0048] In the above step 4), the operating temperature of the water stripper is 20-40° C., and the operating pressure is 0-2 kPa. When the temperature and pressure are too low, the extraction efficiency is low, and when the temperature and pressure are too high, it will cause flooding.

[0049] In the above step 5), the light phase is crude benzene with a caprolactam content of less than 0.1 wt%. If it is greater than 0.1 wt%, the caprolactam content is too high, resulting in a large loss of caprolactam and poor economic efficiency.

[0050] In the above step 5), the heavy phase is a caprolactam-water solution with a caprolactam content of 25wt% to 40wt%; if it is less than 25wt%, the caprolactam content is too low, the water content in the caprolactam-water solution is high, and in order to evaporate water, the amount of steam required for the triple-effect evaporation and distillation process increases; if it is greater than 40wt%, the amount of water added in the stripping process is small, and caprolactam may be entrained in the water phase at the top of the stripping tower, resulting in the loss of caprolactam.

[0051] The method provided by the present invention is further illustrated by the following examples, but is not limited to the following examples.

[0052] Example 1

[0053] This embodiment provides a method for reducing water-soluble impurities in a benzoate-caprolactam solution, comprising the following steps:

[0054] The mixed solvent is prepared by mixing benzene and n-hexane in a mass ratio of 0.1:1;

[0055] Take the benzene-hexyl solution discharged from the light phase at the top of the tower in the primary extraction process, and send it to the extractor for secondary extraction with the above-mentioned mixed solvent at a mass ratio of 1:2, wherein the amount of caprolactam in the benzene-hexyl solution is 20wt%, the secondary extraction temperature is 30°C, and the extraction pressure is normal pressure;

[0056] The light phase after the secondary extraction is distilled, the composition of the light phase is benzene: n-hexane: caprolactam mass ratio = 4:10:1, and the distillation temperature is 72°C; the distillation residue is distilled twice, and the distillation temperature is 85°C;

[0057] The secondary distillation residue was taken for water back extraction at a temperature of 40°C, an operating pressure of 1 kPa, and a mass ratio of benzene-hexyl solution to water of 6:1;

[0058] The mass content, conductivity and extinction value of caprolactam were measured by taking the aqueous solution of caprolactam obtained by water back extraction. The test results are shown in Table 1.

[0059] Table 1 Results of Example 1

[0060]

[0061] The content of water-soluble impurities in the benzene-hexyl solution is reflected by the conductivity and extinction. The lower the conductivity and extinction, the lower the content of water-soluble impurities. From the data in Table 1, it is known that the addition of a mixed solvent for secondary extraction significantly reduces the conductivity of the benzene-hexyl solution, and the extinction value of the benzene-hexyl solution is <1, so it is known that the addition of a mixed solvent reduces the content of water-soluble impurities in the benzene-hexyl solution; it is calculated that the amount of water added in the stripping process is reduced from 33wt% to 14.2wt%, and the concentration of the stripping material hexyl water solution is increased from 30wt% to 55wt%.

[0062] Example 2

[0063] This embodiment provides a method for reducing water-soluble impurities in a benzoate-caprolactam solution, comprising the following steps:

[0064] Prepare a mixed solvent by mixing benzene and n-hexane in a mass ratio of 1:1;

[0065] Take the benzene-hexyl solution discharged from the light phase at the top of the tower in the primary extraction process, and send it to the extractor for secondary extraction with the mixed solvent in a mass ratio of 1:2, wherein the caprolactam content in the benzene-hexyl solution is 20wt%, the secondary extraction temperature is 30°C, and the extraction pressure is normal pressure;

[0066] The light phase after the secondary extraction was distilled, the composition of the light phase was benzene: n-hexane: caprolactam mass ratio = 4:10:1, and the distillation temperature was 72°C; the distillation residue was distilled again, and the distillation temperature was 85°C; the light phase after the secondary extraction was taken to measure the conductivity;

[0067] The secondary distillation residue was taken for water back extraction at a temperature of 40°C, an operating pressure of 1 kPa, and a mass ratio of benzene-hexyl solution to water of 6:1;

[0068] The caprolactam mass content, electrical conductivity and extinction value of the caprolactam aqueous solution obtained by water stripping were measured. The test results are shown in Table 2.

[0069] Table 2 Results of Example 2

[0070]

[0071] From the data in Table 2, it can be seen that adding a mixed solvent for secondary extraction significantly reduces the conductivity of the benzene-hexyl solution, and the extinction value of the benzene-hexyl solution is <1, so it is known that adding a mixed solvent reduces the content of water-soluble impurities in the benzene-hexyl solution; calculation shows that the amount of water added in the stripping process is reduced from 33wt% to 14.2wt%, and the concentration of the stripping material hexyl water solution is increased from 30wt% to 46wt%.

[0072] Example 3

[0073] This embodiment provides a method for reducing water-soluble impurities in a benzoate-caprolactam solution, comprising the following steps:

[0074] Prepare a mixed solvent by mixing benzene and n-hexane in a mass ratio of 0.1:1;

[0075] Take the benzene-hexyl solution discharged from the light phase at the top of the tower in the primary extraction process, and send it to the extractor for secondary extraction with the mixed solvent in a mass ratio of 1:2, wherein the caprolactam content in the benzene-hexyl solution is 20wt%, the secondary extraction temperature is 30°C, and the extraction pressure is normal pressure;

[0076] The light phase after the secondary extraction is distilled, the composition of the light phase is benzene: n-hexane: caprolactam mass ratio = 4:10:1, and the distillation temperature is 72°C; the distillation residue is distilled twice, and the distillation temperature is 85°C;

[0077] The secondary distillation residue was taken for water back extraction at a temperature of 40°C, an operating pressure of 1 kPa, and a mass ratio of benzene-hexyl solution to water of 4:1;

[0078] The caprolactam mass content, conductivity and extinction value of the caprolactam aqueous solution obtained by water back extraction were measured. The test results are shown in Table 3.

[0079] Table 3 Results of Example 3

[0080]

[0081]

[0082] From the data in Table 3, it can be seen that adding a mixed solvent for secondary extraction significantly reduces the conductivity of the benzene-hexyl solution, and the extinction value of the benzene-hexyl solution is <1, so it is known that adding a mixed solvent reduces the content of water-soluble impurities in the benzene-hexyl solution; calculation shows that the amount of water added in the stripping process is reduced from 33wt% to 20wt%, and the concentration of the stripping material hexyl water solution is increased from 30wt% to 40wt%.

[0083] The above embodiments are only used for the technical solutions of the present invention, but not to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it should be understood by those skilled in the art that the technical solutions described in the aforementioned embodiments can still be modified, or some of the technical features can be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for reducing water-soluble impurities in a benzoic acid caprolactam solution, characterized in that: The steps include: 1) sending the benzene-hexyl solution and n-hexane or a mixed solvent of n-hexane and benzene obtained in the primary extraction process into an extractor for secondary extraction; the benzene-hexyl solution comprises benzene and caprolactam and impurities dissolved therein, wherein the water-soluble impurities in the impurities are mainly ammonium sulfate; 2) sending the light phase obtained from the secondary extraction in step 1) to a solvent for primary distillation to distill out the n-hexane for recycling; 3) performing secondary distillation on the benzene-hexyl solution remaining after the primary distillation in step 2) to distill out the benzene for recycling; 4) subjecting the benzene-containing caprolactam solution extracted from the secondary distillation tower bottom in step 3) to water back extraction; 5) The light phase obtained by water stripping in step 4) is subjected to benzene distillation to recover benzene, and the heavy phase obtained by water stripping is sent to benzene stripping, ion exchange, hydrogenation, three-effect evaporation, separation, flash evaporation, light removal, and distillation steps in sequence.

2. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 1, characterized in that: In step 1), the mass content ratio of benzene to caprolactam in the benzene-hexyl solution is 2.5 to 8:

1.

3. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 2, characterized in that: In step 1), the mass content of the water-soluble impurity ammonium sulfate in the benzene-hexyl solution is 0.1-10%.

4. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 1, characterized in that: In step 1), the mixed solvent includes benzene and n-hexane.

5. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 1, characterized in that: In step 1), the mass ratio of benzene to n-hexane in the mixed solvent is 0.1 to 1:

1.

6. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 1, characterized in that: In step 1), the temperature of the secondary extraction is 40° C. and the pressure is 1 kPa.

7. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 1, characterized in that: In step 1), the mass ratio of the mixed solvent to the benzene-hexyl solution is 1:2-10.

8. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 1, characterized in that: In step 2), the operating temperature of the first distillation is 65-75°C; in step 3), the operating temperature of the second distillation is 75-95°C.

9. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 1, characterized in that: In step 4), the mass ratio of benzene to caprolactam in the caprolactam solution is 1:1-4; the mass ratio of the benzene-hexyl solution added to the water stripper is 6-4:1; the operating temperature of the water stripper is 40°C and the operating pressure is 1 kPa.

10. The method for reducing water-soluble impurities in a phenyl caprolactam solution according to claim 1, characterized in that: In step 5), the light phase is crude benzene with a caprolactam content of less than 0.1 wt%; and the heavy phase is a caprolactam-water solution with a caprolactam content of 25 wt% to 40 wt%.

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